1. Field of the Invention
The present invention generally relates to laser fuse blow techniques used in integrated circuit arrays to invoke redundant elements of the array and, more particularly, to an improvement in laser ablation to improve fuse blow yield.
2. Background Description
Laser ablation at normal incidence is currently used to ablate or open electrical lines in many integrated circuit (IC) devices such as dynamic random access memories (DRAMs) or static random access memories (SRAMs) to invoke redundant elements of the array. This is necessary as it can be shown that without redundancy, the manufacturing yields for a complex process as needed for a sixty-four megabyte (64 MB) DRAM device would be very low indeed.
FIG. 1 shows an example of a multilayer stack of dielectric material surrounding an electrical line constituting a fuse which is to be blown. The top film layer 11 is a silicon dioxide (SiO.sub.2) layer having an optical constant defined by its index of refraction n=1.44-0i. Layer 12 is a thin silicon nitride (Si.sub.3 N.sub.4) layer with n=1.99-0i. Layer 13 is an absorbing tungsten silicide layer (WSi), having an n=3.90-1.6i. Layer 14 is a polysilicon layer having n=3.4-0.02i. Finally, layer 15 is another oxide layer which has the same optical constants as layer 11. This stack is assumed to lie on top of a silicon (Si) substrate 16 having "infinite" thickness and n=3.54-0i. All the constants are given for the wavelength of interest, that of a yttrium aluminum garnet (YAG) infrared (IR) laser at a wavelength (.lambda.) of 1.06 micrometer (.mu.m).
Arrow 17 represents incident light energy, while arrows 18, 19 and 20 represent light energy reflected at each interface. The electric and magnetic fields are perpendicular to the direction of the light energy 17. The reflections 18, 19 and 20 at each interface cause an almost 180.degree. reversal of the electric and magnetic fields. This reversal cancels the energy which passes through the multilayer stack.
For such a film stack, incident light energy 17, would show a variation of energy absorbed by the film as a sinusoidal curve which is a function of the thickness of the films. This is calculated easily and shown in FIG. 2. The point of interest is that there is only about 40% absorption of the energy at the thickness of choice. Of course, other thicknesses give higher absorption, but may not be possible from a process design point of view. For instance, higher thicknesses may cause unsuccessful fuse ablation while much thinner thicknesses may not give the appropriate level of electrical or environmental protection needed for high yields and reliability of the device.